1. Field Of The Invention
The present invention relates to a method for positioning a head in a disc recording unit, typically a so-called fixed disc unit.
More particularly, the present invention relates to a method for positioning a head by controlling a phase current of a motor so as to write a plurality of information data on respective circumferential tracks which are concentrically spaced apart from each other at various radial positions on a major surface of a disc or to read out a plurality of information data from the respective tracks.
2. Description Of The Prior Art
The reduction in diameter and the increase in storage capacity of a disc are demanded in a recently developed disc recording unit such as a fixed disc device. In order to meet these demands, efforts have been made to increase the storage density of a disc. It is necessary to provide several hundred recording tracks on each major surface of a small-diameter disc. As a result, a prior art open head control system which has been employed is not satisfactory in practice in order to properly position a transducer head for writing and reading data into and from the surface of the disc at the center line of each of a large number of tracks.
In the prior art control system of the type described above, only a so-called "seek" signal is applied to an actuator for driving the head so that the head is positioned at a designated track and the present position of the head is not detected. Accordingly, fine adjustment of the position of the head is not performed. For instance, in case of a 5.25-inch fixed disc unit, the open control system can be used when the maximum track density is less than 500 TPI (tracks per inch), but when the track density exceeds 700 TPI, a closed loop control system must be used in order to properly position the head in relation to the specific track.
Of course, the closed loop control system is based upon correct detection of the present position of the head. To this end, employed widely is a system in which the head itself reads out so-called servo information recorded on the surface of the disc so that the position of the head is detected by processing the readout signal.
In general, the so-called servo information recorded on the disc is in the form of digital signal like information data stored in a disc, and hence it is necessary to distinguish the readout servo information distinctly from the information data. To distinguish the information from each other, various methods are possible and in one of the methods, a read/write head is used to write the servo information in a region (that is, a track) which is spaced apart in the radial direction of the disc from a track in which the information data is stored.
The control system of the type described above will be further explained in detail with reference to FIGS. 1-4.
FIG. 1 illustrates major component parts of a prior art fixed disc recording unit. In FIG. 1, a plurality of discs 1 are securely attached to a spindle 1a and are spaced apart from each other by a predetermined distance in the vertical direction. The discs 1 are rotated at a high rotational velocity by a spindle motor 1b. One head 4 for writing and reading the data into and from the surface of the disc 1 is allotted to each surface of the discs 1. The respective heads 4 are mounted to head arms 4a which are supported by a carriage 5. An electric motor 6 reciprocates the carriage 5 and hence the heads 4 via a thin metal plate 5b in the directions indicated by an arrow P.
Each surface of the discs 1 is provided with a large number of concentric tracks 2 as best shown in FIG. 2 and so-called servo information 3 is recorded on a region spaced apart from each track 2 in the radial direction and in this case in a sector which interrupts each track 2.
Shown in FIG. 3 is the electric motor 6 for driving the head 4 for reading information data written in the track 2 and the servo information 3 from the surface of the disc 2. The motor 6 in FIG. 3 is a two-phase (in general, multi-phase) stepping motor. In response to the number of driving pulses applied to the motor 6, the head 4 is moved in the radial direction and stopped at a specific track 2 or the region 3 on which the servo information is recorded. In addition, the motor 6 functions as a torque motor in which depending on magnitudes of currents Ia and Ib flowing through coils 6a and 6b of the motor 6, phase position of a rotor 6c is controlled. In this case, the position of the rotor 6c is schematically indicated by a vector angle .theta. of a current vector sum I of the phase currents Ia and Ib. By the fine adjustment of the position of the rotor 6c, the head 4 is accurately brought to the center line of the track 2 or to a correct position at which the servo information 3 is stored.
FIG. 4 shows a detailed positional relationship between the detail of the servo information and the track 2. As is clear from FIGS. 2 and 4, the region 3 on which the servo information is recorded is radially spaced apart from the adjacent track 2 by a distance substantially equal to one half of the pitch between the tracks 2 and in order to detect the position of the head 4 in relation to the specific track 2 as shown in FIG. 4, two areas for servo information 3a and 3b are used. Each of the areas consists of a simple repetitive pattern of digital values as indicated by the vertical line segments. The two adjacent regions 3a and 3b on which the servo information is recorded are displaced from each other in the circumferential direction Q (that is, in the direction of rotation) so that they are not adjacent to each other in the circumferential direction.
When the head 4 is at the normal position PN with respect to the specific track 2, it is apparent that the magnitudes of the signals read out from the servo information 3a and 3b by the head 4 are equal to each other, and consequently the head 4 detects that the head 4 is properly positioned with respect to the specific track 2. However, when the head 4 is deviated from the normal position PN upwardly or downwardly even by a small distance as indicated by the chain lines, the magnitude of the signal read out from one of the two areas 3a and 3b for the servo information becomes higher than the magnitude of the signal read out from the other area for the servo information. Accordingly, the direction of the deviation of the head 4 can be determined depending upon which signal has a larger magnitude. The positional deviation .DELTA.P can be detected from the difference between the magnitudes of the two signals. Therefore, in response to the positional data thus obtained, the head 4 can be so controlled as to be positioned at its normal position PN.
Meanwhile, in order to write the servo information 3a and 3b used for the detection of the position of the head 4, the head 4 must be brought to the positions Pa and Pb deviated from the normal position PN at which the head 4 is properly positioned with respect to the specific track 2. Furthermore, these deviated positions Pa and Pb must be spaced apart from the normal position PN by the same distance. It is of course possible to control the position of the head 4 so as to satisfy the above-described requirement when the servo information 3a and 3b is recorded. For instance, as shown in FIG. 3, in order to position the head 4 at the position Pa or Pb, the phase current Ia or Ib is solely supplied to the motor 6, while in order to position the head at its normal position PN, it is sufficient that the phase currents Ia and Ib having the same magnitude are simultaneously applied to the motor 6.
In practice, however, even when the head 4 is accurately controlled in the manner described above so as to write the servo information 3a and 3b, it occurs frequently that even when the head 4 is properly driven to be positioned at the specific track 2, it is not brought to its normal position PN, but to a position deviated from the normal position PN by .DELTA.P.
Of course, in response to the signals obtained by reading the servo information 3a and 3b, such deviation .DELTA.P can be corrected. But, in the case of a closed loop control system, information data is written into or read out from the specific track 2 only after the head 4 is accurately brought to the normal position PN, so that when the deviation .DELTA.P occurs frequently, there arises the problem that the so-called access time, that is, a time interval from an instant at which a read/write instruction is issued to an instant at which a read/write operation starts, is long.
As described above, when two or more kinds of information such as information data and the servo information are written into or read out from the same surface of a disc, it takes an unexpectedly long time to bring the head to a the proper position for reading or writing by a closed loop control system. That is, there is a tendency that the access time becomes longer. So far, the reasons for such a longer access time have not yet been clarified in detail and there has been an increasing demand for shortening the access time so that the inherent performance of the closed loop control system may be fully utilized.